Despite numerous attempts at making better solar cells with new and exotic materials, the photovoltaics market is still dominated by early or first generation solar cells that are typically silicon wafer-based solar cells. Most solar cell manufacturers are equipped to produce silicon wafer-based solar cells, and research continues to design silicon-based solar cells that can achieve higher conversion efficiencies without an exorbitant increase in production costs, e.g., the aim of research often is to achieve the lowest cost per watt solar cell design that is suitable for commercial production. In addition to use in solar cells, silicon wafers, other silicon layers on substrates, and objects having silicon surfaces are used in numerous other applications such as in electronic devices, telecommunication devices, computers, and even in biological or medical applications, and these applications have also driven research to methods of fabricating silicon wafers and silicon surfaces with particular qualities or characteristics such as a rough, textured, or nanostructured surface.
The performance of solar cells and other optoelectronic devices is directly related to optical losses caused by high reflectivity. Flat silicon surfaces such as those found on an untreated silicon wafer have a high natural reflectivity across the entire range of the solar spectrum that could otherwise be converted to electrical energy by the silicon photovoltaic device. To produce high efficiency solar cells, researchers have sought ways to minimize reflection losses. One common approach has been to provide anti-reflection coatings (ARCs) that typically are selected based on interference. For example, quarter wavelength transparent layers of materials such as SiOx, TiOx, ZnO, ITO, or Si3N4 are used as ARCs on silicon surfaces. In some cases, ARCs from oxidized silicon may be formed by electrochemical etching. All such ARC coatings are resonant structures and perform well only in a limited spectral range and for specific angles of incidence while the solar spectrum spans a wide range of wavelengths and the incident angle varies during the day. The typical results achieved with simple one-layer ARCs have been a reduction of the surface reflection to about 8 to 15 percent. With more difficult two-layer ARC coatings, the reflectivity can be reduced to about 4 percent, but this kind of coating is expensive to apply and is not effective when placed under glass in photovoltaic modules.
Researchers have shown that efficient suppression of reflection in a broad spectral range can be achieved by deep surface texturing. In this regard, etching can be used on a smooth or polished silicon surface to produce rough surfaces with bumps and pits having typical sizes of several or even ten micrometers, and these rough surfaces exhibit reduced reflectivity due to the reflection and absorption characteristics. In one example, anisotropic etching of silicon in KOH/C2H5OH mixtures produces densely packed pyramids that appear black. However, such etching has been typically limited to single crystalline silicon with <1,0,0,> surface orientation, and solar cell design is made more complex by the large penetration pyramids. This texturing also has reflectivity that increases rapidly with the angle of light incidence.
More recently, researchers determined that a fine surface texturing on the nanometer scale may be utilized to control reflectivity of silicon surfaces. Specifically, a textured surface with features smaller than the wavelength of light is an effective medium for controlling reflectivity, and testing with regard to solar cell applications has shown that a fine texture that is only about 300 to 500 nanometers in depth and provides a gradual grading of the silicon density and of the index of refraction from the surface to the bulk that is adequate to suppress reflectivity of a silicon surface in the usable spectral range of photon energies above the band gap. Such a textured surface may be thought of as a subwavelength structured surface that behaves itself as an anti-reflective surface, with the gradually tapered density of the anti-reflective surface suppressing reflection over a wide spectral bandwidth and over a large incidence angle of the incoming light. One group of researchers has developed a method of nanoscale texturing of silicon surfaces that utilizes wet chemical etching to reduce optical losses due to surface reflection to below 5 percent at all solar wavelengths for crystalline silicon.
Briefly, the texturing of the silicon surfaces involves black etching in a three step process. First, a discontinuous gold (Au) layer with a thickness of about 1 to 2 nanometers is deposited by thermal evaporation or other deposition techniques. This initial metal coating is made up of Au clusters or islands that in later steps provide a catalytic action or function. Second, a wet chemical etching of the silicon material is performed using an aqueous solution of hydrofluoric acid (HF) and hydrogen peroxide (H2O2). This solution etches clean or non-coated portions of the silicon surface very slowly but near or about the periphery of the Au islands a texture with a depth of up to 500 nanometers forms very quickly, such as at an etch rate of about 330 nanometers per minute (which indicated catalytic action to these researchers of the Au clusters or islands). Third, since gold is a detrimental impurity in silicon surfaces, the remaining gold is removed from the textured silicon surface such as by room temperature etching in an aqueous solution of iodine and potassium iodide. The researchers indicated that this multi-step process including deposition of a metallic or catalytic layer may be performed on different silicon surfaces including morphologies such as crystalline, multicrystalline, and amorphous as well as differing doping such as n-type, p-type, and intrinsic doping. The amount of absorbed light was increased with this black etch treatment and results showed reflectivity of as little as 2 to 5 percent in the high light absorption ranges of the silicon samples.
While such etching processes produce highly non-reflective or “black” silicon surfaces, there are a number of drawbacks that may hinder wide adoption of such processes. The deposition of gold may be cost prohibitive (e.g., undesirably increase the production cost or price of solar cells or other optoelectronic devices). The costs include material costs associated with deposition of the thin layers of pure gold and also include high capital equipment costs associated with the purchase, operation, and maintenance of vacuum deposition and other equipment used in the metallic deposition steps of the process. The process also requires two or more steps to provide the etching or texturing, which increases manufacturing complexity and fabrication times. Hence, there is an unmet demand for inexpensive, less complex (e.g., processes with fewer steps and less equipment), and efficient techniques for etching silicon surfaces including ways to facilitate black etching of silicon wafers.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.